Method for improving the surface finish of additive manufactured articles

ABSTRACT

An additive elastomeric manufactured part having improved surface finish is made by repeatedly extruding through a nozzle to build up layers of a material comprised of a prepolymer comprised of an isocyanate terminated prepolymer and a filler in an amount such that the material has a shear storage modulus G′ of 100,000 to 300,000 Pa measured at an oscillation rate of 1 Hz and a relaxation time of 20 seconds to 360 seconds. It has been discovered that the particular material having these rheological properties is able to improve the surface finish of the additive manufactured part without slumping and is believed to be due to surface flow of material into valleys between the extrudates as they are being built up.

Field of the Invention

The invention relates to a method of additive manufacturing of thermosetpolymers. In particular, the invention is an additive manufacturingmethod for forming elastomeric parts (e.g., polyurethane) in which theas manufactured article's surface finish is improved.

BACKGROUND OF THE INVENTION

Fused filament fabrication (FFF), which is also commonly called plasticjet printing or fused deposition modeling (FDM) has been used to form 3Dparts by using thermo-plastic filaments that are drawn into a nozzle,heated, melted and then extruded where the extruded filaments fusetogether upon cooling (see, for example, U.S. Pat. Nos. 5,121,329 and5,503,785). Because the technique requires melting of a filament andextrusion, the materials have been limited to thermoplastic polymers(typically nylon) and complex apparatus. The technique has requiredsupport structures that are also extruded when making complex parts thatmust survive the elevated temperature needed to form the part, whilealso being easily removed, for example, by dissolving it. Becausearticles are made by extruding through a nozzle, which typically arecircular in cross-section and have a diameter from about 100 to 200micrometers, the surface finish of the parts tend to be rough and have aperiodicity in the Z or build direction that has required smoothing formany applications.

Recently, in co-pending application PCT/US15/055266 by A. J. Pyzik et.al., a FDM like technique was described in which thermoset materialswere extruded at room temperature to form elastomeric parts. Pyzik et.al., also describe various nozzle cross-sections such as trapezoids aswell as extrudate lay down patterns at least in part to try and improvethe surface finish. Nevertheless, there is still a need to improve thesurface finish of FDM additive manufactured parts.

It would be desirable to provide an FDM additive manufacturing methodand parts made therefrom that avoid one or more of the problems of theprior art such as those described above and in particular the ability tomake parts with a smoother surface finish.

SUMMARY OF THE INVENTION

A first aspect of the invention is a method of additive manufacturingcomprising,

-   -   (i) providing a material comprised of a prepolymer comprised of        an isocyanate terminated prepolymer and a filler in an amount        such that the material has a shear storage modulus G′ of 75,000        to 300,000 Pa and a relaxation time of 20 seconds to 360        seconds,    -   (ii) dispensing said material through a nozzle to form an        extrudate deposited on a base,    -   (iii) moving the base, nozzle or combination thereof while        dispensing the material so that there is horizontal displacement        between the base and nozzle in a predetermined pattern to form        an initial layer of the material on the base, and (iv) repeating        steps (ii) and (iii) to form a successive layer of the material        adhered on the initial layer to form an additive manufactured        part.

The method surprisingly enhances the surface finish of a 3D printedarticle.

The surface finish enhancement may be improved amplitude of peak tovalley heights, overall surface roughness or both. Without beinglimiting, it is believed the extrudates when extruded through a nozzlehaving a diameter of from 100 to 1000 micrometers allows for sufficientsurface flow of the material upon being deposited such that it fills atleast a portion of the valleys created between the extrudates, butwithout causing the article itself to slump. A second advantage is theimprovement in the retention of Z-direction properties (including, forexample, tensile strength and elongation at break) to equal or nearlyequal (within, for example 10%) that of the XY direction.

The improved additive manufacturing method may be used to form anadditive manufactured polymeric part. The method is particularly suitedto make thermoset elastomeric parts such as those used to mitigatenoise, vibration or harshness (NVH) issues in mechanical systems.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side view of the additive manufactured article of thisinvention being made by the method of this invention.

DETAILED DESCRIPTION OF THE INVENTION

The method additive manufacturing involves the use of a materialcomprised of a prepolymer and a filler (filled prepolymer system) wherethe prepolymer generally reacts under the environment it is dispensed toor with a second component simultaneously mixed and dispensed with itand forms a cross-linked or thermoset matrix. Typically, the material isdispensed into an air atmosphere at any useful or suitable temperature.Surprisingly, the material may be dispensed without any heating andretain its shape sufficiently to form an additive manufactured part.Generally, that means at least a portion or all of the prepolymer flowsunder shear at ambient temperature (23° C.). The use of a materialhaving a prepolymer and filler allows for the dispensing of an extrudatethat retains the shape of the nozzle opening that it is extrudedthrough.

The material may be provided as one component or multiple components (2or more). Generally, the material is provided as one component or twoseparate components. When the material is provided as one component, theprepolymer generally reacts in the atmosphere it is dispensed into suchas moisture present in air to form the desired additive manufacturedpart. Illustratively, when the material is provided as two components(separately until dispensed), the components generally react with eachother upon mixing just prior to dispensing to form the desired additivemanufactured part. A component in a material provided in more than onecomponent may have one or more constituents that react with theatmosphere also, but is not required.

When the material is provided as one component, the relative humidity(RH) of the gaseous atmosphere may be used to fine tune the surfacefinish without incurring deleterious slumping. Illustratively, the RHmay be increased if the parts are slumping undesirably or decreased ifthe surface finish is rougher than desired because of the rheologicalproperties being on the edge of the desired ranges of shear storagemodulus (G′) or relaxation time (A). Typically the RH is anywhere from10% to 90% but desirably is from 25% to 75%.

Generally, the material has a high viscosity at low shear to aid in theretention of the shape after being dispensed. “High viscosity” meansthat the viscosity of the material or a component making up the materialis at least about 10,000, 20,000, or 30,000 centipoise to about2,000,000 or 1,000,000 centipoise. It is also preferred that if thematerial is provided in more than one component that each of thecomponents has viscosity that is within about 50% of each othercomponent under the same shear strain rate close to the strain rateexpected to be used to dispense the material. “Near” means the strainrate is ±50% of the strain rate typically used to dispense the reactivematerials. It is even more preferred if the viscosity is within 40%.

A useful indicative low shear measurement is one in which the viscosityis measured using a Brookfield viscometer using a number 5 spindle atthe lowest rpm or using a AR2000 Rheometer available from TAInstruments, New Castle, Del. with a continuous flow method where a 4degree cone plate of 20 mm diameter is used at 25 degree C. along with152 micrometer gap and a shear sweep from 1 to 150 s⁻¹. The viscosity incentipoise at low shear is taken at a shear rate of 5 s⁻¹.

Likewise, the material desirably has a lower viscosity at higher shear(i.e., is shear thinning) to aid in the ease of dispensing. Generally,it is desirable for the material to have a viscosity at 100 s⁻¹ that isat least 2, 3, 5, 10 or even 20 or more times less than at a shear rateof 5 s⁻¹.

It has been discovered that for the material to realize the enhancedsurface finish without slumping, the material must have a particularshear storage modulus (G′) of 100,000 to 300,000 Pa and relaxation time(λ) of 20 to 360 seconds. In measuring G′, the material is first mixedat high shear such as mixing in a container with paddle blades rotatingat 200 rpm for about 1 minute. The material is then placed in arheometer (e.g., AR2000 rheometer from TA Instruments) and anoscillatory stress sweep from 1 to 1000 Pa at a frequency of 1 Hz at 25°C. is performed. A suitable measuring device geometry is a 25 mmparallel plate having a gap of about 1,000 micrometers. Prior toperforming the sweep, a dynamic pre-shear is used to mitigate anyresidual normal force caused by setting the gap of the parallel plate. Asuitable dynamic pre-shear consists of a 0.01 rad displacement at afrequency of 1 Hz for about 1 min. G′ is reported from the linearviscoelastic region. From the stress sweep the yield strength at thecrossover point of G′ and G″ (shear loss modulus) was determined.Desirably, G′ is from 90,000 to 200,000 or 150,000 Pa.

In measuring λ, a stress of 2× the yield strength (determined by thestress sweep above) was applied for 60 minutes in a 25 mm parallel plateconfiguration with a gap of 1 millimeter. Then, a time sweep at 1 rad/s,with an oscillatory stress at 5 Pa was run for 60 minutes whilemeasuring the strain. When the creep stress is removed, the strainrecovery can be measured. Relaxation time is calculated using the TAInstruments Rheology Data Advantage software. Desirably, λ is 30, 50,100 or 125 seconds to 300, 200 or 175 seconds. The software in essencemodels the strain recovery by an exponential equation such as shown inthe following equation:

γ(t)=a+b[e ^(−t/λ)]

where γ is the strain, a is the non-reversible deformation due to creep,and b is a material coefficient and λ is approximated as the time ittakes for the strain to relax to 1/e of it's initial value after theapplication of the stress, or ˜63.2% recovery.

It has been discovered to achieve the desirable rheological propertiesdescribed above, the material is comprised of a prepolymer and filler,with the prepolymer being an isocyanate terminated prepolymer. Theamount of isocyanate is present in a sufficient quantity to provideadhesive character between the extrudates during the formation of theadditive manufactured part. Such prepolymers also have an averageisocyanate functionality sufficient to allow the preparation of acrosslinked polyurethane upon dispensing, but and not so high that thepolymers are unstable. “Stability” in this context means that thematerial prepared from the prepolymer has a shelf life of at least threemonths at ambient temperature, in that it does not demonstrate anincrease in viscosity during such period which prevents its dispensing,application or use. For example, the viscosity should not rise toogreatly to make it impractical to dispense. Preferably, the materialdoes not undergo an increase in viscosity of more than about 50 percentduring the stated period.

The prepolymer of the material desirably has a total NCO content whichfacilitates acceptable strength in parts prepared after 60 minutes andstability of the prepolymer. Total NCO content includes the NCOs fromthe isocyanate terminated prepolymer or unreacted isocyanates used tomake the prepolymers. Preferably, the NCO content is about 0.6 percentby weight or greater based on the weight of the prepolymer and morepreferably about 0.9 percent by weight or greater, and preferably about4.0 percent by weight or less, more preferably about 3.5 percent byweight or less, even more preferably about 3.0 percent by weight orless, and even more preferably about 2.6 percent by weight or less.Below about 0.6 percent by weight, the prepolymer viscosity may be toohigh to handle and the working time may be too short even ifdispensable.

The prepolymer exhibits a viscosity, which facilitates forming amaterial when mixed with the filler having the aforementioned rheology.Generally, the viscosity of the prepolymer is about 100,000 centipoise(100 Pa s) or less and more preferably about 50,000 centipoise (50 Pa s)or less, and most preferably about 30,000 centipoise (30 Pa s) or lessand about 1,000 centipoise (1 Pa s) or greater. The viscosity usedherein is Brookfield viscosity determined using a number 5 spindle.Below about 1,000 centipoise (1 Pa s), the material prepared from theprepolymer may exhibit poor properties or the desired material rheologymay be difficult to achieve. Above about 100,000 centipoise (100 Pa s)the prepolymer may be unstable and subject to very short pot lifes.

The prepolymers of the invention may have any suitable molecular weightsuch as an average between 10,000 to about 1,000,000 g/mole. The“molecular weight average” used herein is the z average molecular weight(Mz) molecular weight average as defined on page 206 of Textbook ofPolymer Science 3rd Edition, Billmeyer, F. W. Jr., John Wiley and Sons,NY, N.Y., 1984. Desirably, the Mz average is at least in ascendingdesirability: 20,000, 30,000, 40,000, 50,000 and 55,000 to at most about1,000,000, 750,000, 500,000, 400,000 or at most about 300,000. Lower Mz(i.e., less than 100,000 g/mole) may be desirable when the materialcontains a reactive silicon component described below.

Preferable polyisocyanates for use in preparing the illustrativeprepolymer include those disclosed in U.S. Pat. No. 5,922,809 at col. 3,line 32 to column 4, line 24, incorporated herein by reference.Preferably, the polyisocyanate is an aromatic or cycloaliphaticpolyisocyanate such as diphenylmethane-4,4′-diisocyanate, isophoronediisocyanate, tetramethylxylene diisocyanate, and is most preferablydiphenylmethane-4,4′-diisocyanate. The diols and triols are genericallyreferred to as polyols.

The prepolymers are made from isocyanate reactive compounds, butpreferably are made using polyols such as diols and triols such as thosedescribed in U.S. Pat. No. 5,922,809 at column 4, line 60 to column 5,line 50, incorporated herein by reference. The polyols (diols andtriols) are polyether polyols and more preferably polyoxyalkylene oxidepolyols. The most preferred triols are ethylene oxide-capped polyolsprepared by reacting glycerin with propylene oxide, followed by reactingthe product with ethylene oxide.

Preferably, the polyether is chosen to decrease the polarity of theprepolymer. A significant factor in determining the polarity of theprepolymer is the amount of ethylene oxide units in the polyether usedto prepare the prepolymer. Preferably, the ethylene oxide content in theprepolymer is about 3 percent by weight or less, more preferably about1.2 percent by weight or less and most preferably about 0.8 percent byweight or less. As used herein “polarity” refers to the impact of thepresence of polar groups in the backbone of the prepolymer. It is alsounderstood that a small amount of other polyols may be used to form thepolyether prepolymer such as a polyester polyol such as those known inthe art. Typically, such other polyols may be present in an amount ofabout up to 5% by weight of the polyols used to make said prepolymer.However, said prepolymer may be made in the absence of such polyols.

The material is also comprised of a filler that assists in the impartingof the desired rheological properties described above. An illustrativefiller that is suitable is a carbon black or filler having similarcharacteristics (e.g., fumed silica), which are as follows.

Depending on their structure and the molecular weight of theprepolymers, the carbon black or filler having similar characteristicsthat may be used may range over a wide range of structures as given byoil absorption number (ASTM D-2414-09). For example, the fillerdesirably has an oil absorption number (OAN) of about 80 to 200 ccs per100 grams, when the Mz of the prepolymer is about 65,000. Preferably,the oil absorption of the filler is at least about 90, more preferablyat least about 100, and most preferably at least about 110 to preferablyat most about 180, more preferably at most about 165 and most preferablyat most about 150 ccs/100 grams.

In addition the filler desirably has an iodine number that is at least80. The iodine number is related to the surface area of the filler, butalso to the presence of volatile species such as unsaturated oils and,sulfur containing compounds in the case of carbon blacks. The iodinenumber is determined using ASTM D1510-11.

Even though it is not understood, it has been discovered that even whenthe oil absorption number is lower than 80ccs/100 grams, the materialmay achieve the desired rheological properties useful in the method ofthis invention. For example, the material may not display sag when theproduct of the OAN and iodine number of the filler is generally at least6,000. Preferably, the product of the OAN (cc/100 g) and iodine number(mg/g) is in rising preference at least 7,000; 8,000; 9,000; 10,000;11,000; 12,000; 13,000 to at most practically obtainable such as 50,000.

The amount of filler (typically carbon black) suitable may be determinedfor a given filler and prepolymer molecular weight, by routineexperimentation. Typically, the amount of filler is at least inascending desirability, 10%, 15%, 16%, or 17% to, ascending 50%, 40%,35%, 30% or 25% by weight of the material.

When a carbon black is used, it may be a standard carbon black which isnot specially treated to render it nonconductive. Standard carbon blackis carbon black which is not specifically surface treated or oxidized.Alternatively, one or more nonconductive carbon blacks may be usedexclusively or in conjunction with the standard carbon black. Suitablestandard carbon blacks include RAVEN™ 790, RAVEN™ 450, RAVEN™ 500,

RAVEN™ 430, RAVEN™ 420 and RAVEN™ 410 carbon blacks available fromColombian and CSX carbon blacks such as ELFTEX 55100 and 57100 andMONARCH 120, 570, and 590 available from Cabot, and PRINTEX™ 30 carbonblack available from Evonik Industries, Mobile, Ala. Suitablenon-conductive carbon blacks include RAVEN™ 1040 and RAVEN™ 1060 carbonblack available from Colombian Chemicals Company, Marietta, Ga.

The material may also be comprised of reactive silicon. The reactivesilicon may be present as a separate molecule such as a silane. It maybe present within the backbone or as a terminal group in the prepolymerdescribed above. The reactive silicon, generally is one that can undergohydrolysis such as described at column 4, lines 25-55 of U.S. Pat. No.6,613,816. Other illustrative reactive silicons may be found in U.S.Patent Publication 2002/0100550 paragraphs 0055 to 0065 and Hsieh, U.S.Pat. No. 6,015,475, column 5, line 27 to column 6, line 41, incorporatedherein by reference.

The amount of reactive silicon, when present in the material is,generally, about 0.001% to 2% by weight of the total weight of thematerial regardless of whether it is provided in one component or more.The amount of the reactive silicon (note, the weight of the siliconitself and does not include, for example, the organic groups appendedthereto), may be at least 0.005%, 0.01%, 0.02%, 0.04%, 0.06%, 0.08% or0.1% to at most 1.8%, 1.6%, 1.4%, 1.2%, 1%, 0.8%, 0.5% of the material.

The material may also be comprised of one or more organic based polymersdispersed therein. Preferably, the organic based polymer is included inthe prepolymer by inclusion of a dispersion triol having dispersedtherein particles of an organic based polymer. Dispersion triolstypically understood to have at least a portion of the particles beinggrafted with the polyol. The preferable dispersion triols are disclosedin Zhou, U.S. Pat. No. 6,709,539 at column 4, line 13 to column 6, line18, incorporated herein by reference. Preferably, the triol used todisperse the organic particles is a polyether triol and more preferablya polyoxyalkylene based triol. Preferably, such polyoxyalkylene oxidetriol comprises a polyoxypropylene chain with a polyoxyethylene end cap.Preferably, the triols used have a molecular weight of about 4,000 orgreater, more preferably about 5,000 or greater and most preferablyabout 6,000 or greater. Preferably, such triol has molecular weight ofabout 8,000 or less and more preferably about 7,000 or less. It isunderstood that the polyol of the dispersion polyol (e.g., triol) isincluded in the polyol to make the prepolymer described herein, wherethe copolymer particles of the dispersion polyol are understood to befillers in the composition.

Preferably, the particles dispersed in the dispersion triol comprise athermoplastic polymer, rubber-modified thermoplastic polymer or apolyurea dispersed in a triol. The polyurea preferably comprises thereaction product of a polyamine and a polyisocyanate. Preferablethermoplastic polymers are those based on monovinylidene aromaticmonomers and copolymers of monovinylidene aromatic monomers withconjugated dienes, acrylates, methacrylates, unsaturated nitriles ormixtures thereof. The copolymers can be block or random copolymers. Morepreferably, the particles dispersed in the triol comprise copolymers ofunsaturated nitriles, conjugated dienes and a monovinylidene aromaticmonomer, a copolymer of an unsaturated nitrile and a monovinylidenearomatic monomer or a polyurea. Even more preferably, the particlescomprise a polyurea or polystyrene-acrylonitrile copolymer with thepolystyrene-acrylonitrile copolymers being most preferred. The organicpolymer particles dispersed in the triol preferably have a particle sizewhich is large enough to improve one or more properties such as impactproperties and elastomeric properties of the finally cured additivemanufactured part. The particles may be dispersed in the triol orgrafted to the backbone to at least a portion of the triols if not allof them. Preferably, the particle size is about 10 micrometers orgreater and more preferably the particle size is about 20 micrometers orgreater.

The polyols are present in an amount sufficient to react with most ofthe isocyanate groups of the isocyanates leaving enough isocyanategroups to correspond with the desired free isocyanate content of theprepolymer. Preferably, the polyols are present in an amount of about 30percent by weight or greater based on the prepolymer, more preferablyabout 40 percent by weight or greater and most preferably about 55percent by weight or greater. Preferably, the polyols are present in anamount of about 75 percent by weight or less based on the prepolymer,more preferably about 65 percent by weight or less and most preferablyabout 60 percent by weight or less.

Generally, the material incorporating the illustrative prepolymertypically has a ratio of diols to triols and dispersion triols toachieve the desired cure rate and strength of the material forming theadditive manufactured part. The weight ratio of diol to triol anddispersion triol, if present, is preferably about 0.8 or greater andmore preferably about 0.85 or greater and most preferably about 0.9 orgreater. The weight ratio of diol to triol and dispersion triol, ifpresent, is preferably about 3.0 or less; more preferably about 2.0 orless and most preferably about 1.75 or less. In the embodiment where thepolyols comprise a mixture of diols and triols, the amount of diolspresent is preferably about 15 percent by weight or greater based on theprepolymer, more preferably about 25 percent by weight or greater andmost preferably about 28 percent by weight or greater; and about 40percent by weight or less based on the prepolymer, more preferably about35 percent by weight or less and most preferably about 30 percent byweight or less. In the embodiment where the polyols comprise a mixtureof diols and triols, the total amount of triols (non-dispersion trioland dispersion triol) present is preferably about 15 percent by weightor greater based on the prepolymer, more preferably about 18 percent byweight or greater and most preferably about 20 percent by weight orgreater; and preferably about 45 percent by weight or less based on theprepolymer, more preferably about 35 percent by weight or less and mostpreferably about 32 percent by weight or less.

The dispersion of organic polymer particles in a triol may be present inthe prepolymer in an amount of about 10 percent by weight or greater ofthe prepolymer and more preferably about 12 percent by weight orgreater, and about 18 percent by weight or less of the prepolymer andmore preferably about 15 percent by weight or less.

The material may further comprise a plasticizer. The plasticizers may beused so as to modify the rheological properties to a desiredconsistency. Such materials should be free of water, inert to isocyanategroups. The plasticizers may be common plasticizers useful inpolyurethane and well known to those skilled in the art and are referredhereinafter as low polar plasticizers. The plasticizer is present in anamount sufficient to disperse the prepolymer of material. Theplasticizer can be added to the prepolymer either during preparation ofthe prepolymer or during compounding of the prepolymer prior to beingplaced into the first compartment. Preferably, the plasticizer ispresent in about 1 percent by weight or greater of the prepolymerformulation (prepolymer plus plasticizer), more preferably about 20percent by weight or greater and most preferably about 30 percent byweight or greater. Preferably, the plasticizer is present in about 45percent by weight or less of the prepolymer formulation and morepreferably about 35 percent by weight or less.

Preferably two plasticizers are used, with one being a high polarplasticizer and one being a low polar plasticizer. A high polarplasticizer is a plasticizer with a polarity greater than the polarityof the aromatic diesters, such as the phthalate esters. A low polarplasticizer is a plasticizer which has a polarity the same as or lessthan the aromatic diesters.

Suitable high polar plasticizers include one or more of alkyl esters ofsulfonic acid, alkyl alkylethers diesters, polyester resins, polyglycoldiesters, polymeric polyesters, tricarboxylic esters, dialkyletherdiesters, dialkylether aromatic esters, aromatic phosphate esters, andaromatic sulfonamides. More preferred high polar plasticizers includearomatic sulfonamides, aromatic phosphate esters, dialkyl ether aromaticesters and alkyl esters of sulfonic acid. Most preferred high polarplasticizers include alkyl esters of sulfonic acid andtoluene-sulfamide. Alkyl esters of sulfonic acid include alkylsulphonicphenyl ester available from Lanxess under the trademark MESAMOLL.Aromatic phosphate esters include PHOSFLEX™ 31 L isopropylated triphenylphosphate ester, DISFLAMOLL™ DPO diphenyl-2-ethyl hexyl phosphate, andDISFLAMOL™ TKP tricresyl phosphate. Dialkylether aromatic esters includeBENZOFLE™ 2-45 diethylene glycol dibenzoate. Aromatic sulfonamidesinclude KETJENFLE™ 8 o and p, N-ethyl toluenesulfonamide

Suitable low polar plasticizers include one or more aromatic diesters,aromatic triesters, aliphatic diesters, epoxidized esters, epoxidizedoils, chlorinated hydrocarbons, aromatic oils, alkylether monoesters,naphthenic oils, alkyl monoesters, glyceride oils, parraffinic oils andsilicone oils. Preferred low polar plasticizers include alkylphthalates, such as diisononyl phthalates, dioctylphthalate anddibutylphthalate, partially hydrogenated terpene commercially availableas “HB-40”, epoxy plasticizers, chloroparaffins, adipic acid esters,castor oil, toluene and alkyl naphthalenes. The most preferred low polarplasticizers are the alkyl phthalates.

The amount of low polar plasticizer in the material is that amount whichgives the desired rheological properties. The amounts disclosed hereininclude those amounts added during preparation of the prepolymer andduring compounding of the material. Preferably, low polar plasticizersare used in an amount of about 5 parts by weight or greater based on theweight of material, more preferably about 10 parts by weight or greater,and most preferably about 18 parts by weight or greater. The low polarplasticizer is preferably used in an amount of about 40 parts by weightor less based on the total amount of material, more preferably about 30parts by weight or less and most preferably about 25 parts by weight orless.

The amount of high polar plasticizer in material is that amount whichgives the desired rheological properties and the acceptable sag andstring properties of the dispensed reactive materials. Preferably, thehigh polar plasticizers are used in the material in an amount of about0.2 parts by weight or greater based on the weight of material, morepreferably about 0.5 parts by weight or greater, and most preferablyabout 1 part by weight or greater. The high polar plasticizer ispreferably used in an amount of about 20 parts by weight or less basedon the total amount of the material, more preferably about 12 parts byweight or less and most preferably about 8 parts by weight or less.

The prepolymer may be prepared by any suitable method, such as byreacting polyols, such as diols, triols and optionally dispersion triolssuch as a copolymer polyol or grafted triol, with an excess overstoichiometry of one or more polyisocyanates under reaction conditionssufficient to form a prepolymer having isocyanate functionality and freeisocyanate content which meets the criteria discussed above. In apreferable method used to prepare the prepolymer, the polyisocyanatesare reacted with one or more diols, one or more triols and, optionally,one or more dispersion triols. Preferable processes for the preparationof the prepolymers are disclosed in U.S. Pat. No. 5,922,809 at column 9,lines 4 to 51, incorporated herein by reference. The prepolymers arepresent in an amount sufficient such that when the resulting dispensedmaterial dispensed and cure, the additive manufactured part is formed bythe method. Preferably, the polyurethane prepolymers are present in anamount of about 20 parts by weight of the material or greater, morepreferably about 30 parts by weight or greater and most preferably about35 parts by weight or greater. Preferably, the prepolymers are presentin an amount of about 60 parts by weight of the material or less, morepreferably about 50 parts by weight or less and even more preferablyabout 45 parts by weight or less.

The material may further comprise a polyfunctional isocyanate, forexample, to improve the modulus of the composition in the cured form oradhesion of the extrudates to each other. “Polyfunctional” as used inthe context of the isocyanates refers to isocyanates having afunctionality of 2 or greater. The polyisocyanates can be any monomeric,oligomeric or polymeric isocyanate having a nominal functionality ofabout 2.5 or greater. More preferably, the polyfunctional isocyanate hasa nominal functionality of about 2.7 or greater. Preferably, thepolyfunctional isocyanate has a nominal functionality of about 5 orless, even more preferably about 4.5 or less and most preferably about3.5 or less. The polyfunctional isocyanate can be any isocyanate whichis reactive with the isocyanate polyisocyanate prepolymers used in thecomposition and which improves the modulus of the cured composition. Thepolyisocyanates can be monomeric; trimeric isocyanurates or biurets ofmonomeric isocyanates; oligomeric or polymeric, the reaction product ofseveral units of one or more monomeric isocyanates. Examples ofpreferred polyfunctional isocyanates include trimers of hexamethylenediisocyanate, such as those available from Bayer under the trademark anddesignation DESMODUR N3300 and N100, and polymeric isocyanates such aspolymeric MDI (methylene diphenyl diisocyanates) such as those marketedby The Dow Chemical Company under the trademark of PAPI, including PAPI20 polymeric isocyanate. The polyfunctional isocyanates, when presentare typically present in an amount sufficient to impact the modulus ofthe cured compositions of the invention or improve the adhesion tocertain substrates described above. The polyfunctional isocyanate, whenpresent, is preferably present in an amount of about 0.5 parts by weightor greater based on the weight of the material, more preferably about1.0 parts by weight or greater and most preferably about 2 parts byweight or greater. The polyfunctional isocyanate is preferably presentin an amount of about 8 parts by weight or less, based on the weight ofthe material, more preferably about 5 parts by weight or less and mostpreferably about 4 parts by weight or less.

The material may also contain a catalyst which catalyzes the reaction ofisocyanate moieties with water or an active hydrogen containingcompound, which may be in a second component. Such compounds are wellknown in the art. The catalyst can be any catalyst known to the skilledartisan for the reaction of isocyanate moieties with water or activehydrogen containing compounds. Among preferred catalysts are organotincompounds, metal alkanoates, and tertiary amines Mixtures of classes ofcatalysts may be used. A mixture of a tertiary amine and a metal salt ispreferred. Even more preferred are tertiary amines, such as dimorpholinodiethyl ether, and a metal alkanoate, such as bismuth octoate. Includedin the useful catalysts are organotin compounds such as alkyl tinoxides, stannous alkanoates, dialkyl tin carboxylates and tinmercaptides. Stannous alkanoates include stannous octoate. Alkyl tinoxides include dialkyl tin oxides, such as dibutyl tin oxide and itsderivatives. The organotin catalyst is preferably a dialkyltindicarboxylate or a dialkyltin dimercaptide. Dialkyltin dicarboxylateswith lower total carbon atoms are preferred as they are more activecatalysts in the compositions of the invention. The preferred dialkyldicarboxylates include 1,1-dimethyltin dilaurate, 1,1-dibutyltindiacetate and 1,1-dimethyl dimaleate. Preferred metal alkanoates includebismuth octoate or bismuth neodecanoate. The organotin or metalalkanoate catalyst is present in an amount of about 60 parts per millionor greater based on the weight of the material, and more preferably 120parts by million or greater. The organotin catalyst is present in anamount of about 1.0 percent or less based on the weight of the material,more preferably 0.5 percent by weight or less and most preferably 0.1percent by weight or less.

Useful tertiary amine catalysts include dimorpholinodialkyl ether, adi((dialkylmorpholino)alkyl) ether, bis-(2-dimethylaminoethyl)ether,triethylene diamine, pentamethyldiethylene triamine,N,N-dimethylcyclohexylamine, N,N-dimethyl piperazine 4-methoxyethylmorpholine, N-methylmorpholine, N-ethyl morpholine and mixtures thereof.A preferred dimorpholinodialkyl ether is dimorpholinodiethyl ether. Apreferred di((dialkylmorpholino)alkyl) ether is(di-(2-(3,5-dimethylmorpholino)ethyl)ether). Tertiary amines arepreferably employed in an amount, based on the weight of the material ofabout 0.01 parts by weight or greater, more preferably about 0.05 partsby weight or greater, even more preferably about 0.1 parts by weight orgreater and most preferably about 0.2 parts by weight or greater andabout 2.0 parts by weight or less, more preferably about 1.75 parts byweight or less, even more preferably about 1.0 parts by weight or lessand most preferably about 0.4 parts by weight or less.

The material may be formulated with other optional components than thosedescribed above. By the addition of such materials, physical propertiessuch as viscosity flow rates and the like can be modified. However, toprevent premature hydrolysis of the moisture sensitive groups of thepolyurethane prepolymer, fillers should be thoroughly dried beforeadmixture therewith. Optional components for use in the material includeoptional other fillers and pigments. Such fillers may include, forexample, titanium dioxide, aluminum oxide, zeolite, calcium carbonate,silica, titanium oxide, silica, talc, pigments and the like. In oneembodiment, more than one other filler may be used. The fillers aretypically used in an amount sufficient to increase one or more desiredproperty such as strength of the additive manufactured part or impart aparticular color.

Other optional fillers may include clays. Preferred clays includekaolin, surface treated kaolin, calcined kaolin, aluminum silicates andsurface treated anhydrous aluminum silicates. The clays can be used inany form, which facilitates formulation of a dispensable material.Preferably, the clay is in the form of pulverized powder, spray-driedbeads or finely ground particles. Clays may be used in an amount ofabout 0.1 parts by weight of the material or greater, more preferablyabout 12 parts by weight or greater and even more preferably about 18parts by weight or greater. Preferably, the clays are used in an amountof about 30 parts by weight or less of the material, more preferablyabout 28 parts by weight or less and most preferably about 24 parts byweight or less.

The material may further comprise stabilizers, which function to protectthe prepolymer from moisture, thereby inhibiting advancement andpreventing premature crosslinking of the isocyanates in the material.Stabilizers known to the skilled artisan for moisture curingpolyurethane compositions may be used. Included among such stabilizersare diethylmalonate, alkylphenol alkylates, paratoluene sulfonicisocyanates, benzoyl chloride and orthoalkyl formates. Such stabilizersare preferably used in an amount of about 0.1 parts by weight or greaterbased on the total weight of the material, preferably about 0.5 parts byweight or greater and more preferably about 0.8 parts by weight orgreater. Such stabilizers are used in an amount of about 5.0 parts byweight or less based on the weight of the material, more preferablyabout 2.0 parts by weight or less and most preferably about 1.4 parts byweight or less.

The material when it is comprised of a second component may be any thatreacts the prepolymer of a first component such as described abovewherein the first component is comprised of the illustrative isocyanateterminated prepolymer such as those containing reactive hydrogens suchas the polyols described above or water.

In one embodiment, the second component is a paste containing water or areactive constituent that enhances the cure of the first component ofthe material. A paste containing water or reactive constituent ispresent to speed up the cure of the material of the first component(i.e., reacts with the isocyanate groups in the first component). Theuse of such a paste is particularly useful when making larger parts thatneed to support more weight upon being formed. Examples of such secondcomponents that react with isocyanate prepolymers are described bycommonly owned copending U.S. application No. 61/990136 having aninventor Lirong Zhou and WO/2014/098935, each incorporated herein byreference. In a particular embodiment, the second component is comprisedof a polyol having a backbone comprised of an amine group, which isfurther described in U.S. application No. 61/990136.

In another embodiment of a two component system, the material iscomprised of an acrylate monomer with a catalyst for forming apolyacrylic or polyacrylate are in two separate components making up thematerial. Said material undergoes two modes of curing to form theadditive manufactured part. Exemplary materials having such twocomponents are described by U.S. Publ. No. 2012-0279654 Int. Pub. Nos.WO/2012/151085 and WO/2012/087490.

The use of a material having 2 components may be desirable, for example,when making larger parts or faster fabrication and use is desired due tothe faster increase in the modulus as the material cures. Generally, theelastic modulus is at least 0.1 MPa upon fully curing to any usefulmodulus, but generally is less than about 50 MPa. Desirably when makingan elastomeric additive manufactured part, the fully cured modulus is atleast about 0.5 MPa or 1 MPa to at most about 25 MPa, 10 MPa, or 5 MPa.The modulus may be determined by the method described by ASTM D4065measured at 25° C. Desirably, 50% of the final cure is obtained in lessthan a couple of days. Preferably, 50% cure is obtained in less than aday, 12 hours, 3 or 4 hours, 1 hour or even 30 minutes.

Turning to FIG. 1, the method comprises dispensing the mixture throughnozzle 100 attached to the nozzle assembly 110 where the mixture may bemixed in-line if it is provided in more than one component. Upondispensing the mixture forms an extrudate 120 that forms an initiallayer 130 and successive layers 140 on base 150. Nozzle assembly 110 isdepicted being orthogonal to base, but may be set at any useful angle toform the extrudate whereby the extrudate 120 and nozzle assembly 110form an obtuse angle with the extrudate 120 being parallel to the base.In addition, the nozzle assembly 110 may be rotated about itslongitudinal axis, for example, to reorient the shape of the opening inthe nozzle 100, to create extrudates 120 having differing relationshipto the base 150.

The relative motion of the base 150 and nozzle assembly 110 are alsoshown, but it is understood that the base 150, nozzle assembly 110 orboth may be moved to cause the relative motion in any horizontaldirection or vertical direction. The motion is made in a predeterminedmanner, which may be accomplished by any known CAD/CAM methodology andapparatus such as those well known in the art and readily availablerobotics or computerized machine tool interface. Such pattern forming isdescribed, for example, in U.S. Pat. No. 5,121,329.

The extrudate 120 may be dispensed continuously or disrupted to form theinitial layer 130 and successive layers 140. If disrupted extrudates 120are desired, the nozzle may be comprised of a valve (not pictured) toshut off the flow of the material. Such valve mechanism may be anysuitable such as any known electromechanical valves that can easily becontrolled by any CAD/CAM methodology in conjunction with the pattern.

When the mixture is comprised of more than one component, the nozzleassembly 110 may also be comprised of a mixer such as an in-line staticor dynamic mixer as well as separate compartments to hold the twocomponents. Examples of two component dispensing apparatus and methodsthat may be suitable include those described in U.S. Pat. Nos. 6,129,244and 8,313,006 and copending U.S. Appl. No. 61/977668 having an inventorHuide Zhu as well as those described by Sulzer Chemtech, Mixpac PeelerII product Brochure and by Craig Blum, Two Component Adhesive CartridgeSystems, FAST, July 2008.

Because the mixture may be adhesive, the base 150 may be a low surfaceenergy material such as a polyolefin (e.g., polyethylene orpolypropylene) or fluorinated polymer such as Teflon and the like.Alternatively, the base may have a mold release agent such as thoseknown in the polyurethane reaction injection molding art or the base mayhave a sheet of paper or film of a low energy material placed upon itprior to dispensing and forming the additive manufactured part.

More than one nozzle assembly 110 may be employed to make composite orgradient structures within the additive manufactured part. Likewise, asecond nozzle assembly 110 may be employed to dispense a supportstructure that may be later removed so as to allow more complexgeometries to be formed such as described in U.S. Pat. No. 5,503,785.The support material may be any that adds support and be removed easilysuch as those known in the art, for example, waxes.

The method surprisingly may be used to make thermoset elastomericadditive manufactured parts. “Elastomeric” means that the additive partdisplays rubber like qualities such as at least about 50% elongationprior to break in tension. Preferably, the elongation at break undertension is at least 100%, 200% or even 300%. In a particular embodimentof such an additive manufacture part, the thermoset elastomeric additivemanufactured part is comprised of polyurethane having a filler whereinthe product of the oil absorption number (cc/100 g) and iodine number(mg/g) is in rising preference at least 7,000; 8,000; 9,000; 10,000;11,000; 12,000; 13,000 to at most practically obtainable such as 50,000.

EXAMPLES Prepolymer Preparation:

A polyether isocyanate terminated polyurethane prepolymer was preparedas described in Comparative Example 6 of U.S. Pat. No. 8,729,168.

Example 1

The material for 3D printing was made by mixing 30 g of prepolymer and 6g filler (ELFTEX™ 57100 Carbon Black available from Cabot Corp.), 17.1%by weight, 2000 RPM for 2 minutes using a DAC 400 Speed Mixer (FlackTekInc, Landrum S.C.) and then further mixing in 0.35 g catalyst2,2′-dimorpholinodiethylether (DMDEE) for 2 more minutes. The filler hada OAN of about 117 cc/100 g and Iodine number of 189 mg/g. The materialwas then transferred into a plastic bag, and extruded into a 10 ccsyringe barrel, plugged with a white Smoothflow piston, and capped withan EFD snap-on endcap, all purchased from Nordson Corporation, WestlakeOhio. The G′, λ and viscosity were determined as described above and aregiven in Table 1.

A high pressure dispensing tool, Nordson HP4X, Nordson Corporation,Westlake Ohio, was mounted on an UltraTT EFD automated dispensingsystem, (Nordson Corporation, Westlake Ohio) which acts as aprogrammable XYZ stage. The filled syringe was loaded into the dispenserand the material pushed through a 0.41 mm luer lok tapered nozzle(7005009, Nordson Corporation. Westlake Ohio) extruded as a circularextrudate on Synaps Digital XM Polyester-Coated paper (Nekoosa CoatedProducts, Nekoosa Wis.) laying on the XYZ table. The material wasextruded at speed of 15 mm/sec using 20 psi air pressure into 35% RHair. The XYZ table was controlled by a PalmPilot to form single-walledsquare tubes with side dimensions of 50 mm 40 layers of extrudates wereprinted in the Z-direction with a step height between layers of 0.20 mmAfter printing was completed, the part was removed (together with papersubstrate) and allowed to cure in the 35% RH air. No delaminationbetween individual layers was observed and adhesion was very good. Nobuckling of build walls or deformation of individual layers wasobserved. The tensile strength in the build direction (z) andperpendicular to the build direction (xy) as well as the correspondingtensile elongation in these directions were determined from ASTM D412standard performed on a Texture Technologies TA-XT-Plus Texture analyzer(Texture Technologies Corp., Hamilton, Mass.) and are shown in Table 2.

The surface finish of the additive manufactured article was determinedusing a Veeco Wyko NT9100 Optical profiling system (Veeco, PlainviewN.Y.) utilizing Scanning White Light Interferometery (SWLI). Thenon-contact three dimensional surface characteristic of surfaceroughness (Ra) was measured using a 50× objective with 1× field of viewwith scanning set points of 50 micrometer backscan, 150 micrometer scanlength, and 2% modulation. A stitching profile was used to combineseveral scans to an array consisting of 127 micrometers×900 micrometers.The surface roughness (Ra) of a sample consisted of an average surfaceroughness of nine arrays (127 micrometer×900 micrometer) in which thetilt and curvature terms were removed in the data analysis. The peak tovalley measurements across the printed rows were measured from theindividual arrays.

Comparative Examples 1-3

Comparative Examples 1-3 were made in the same way as Example 1, but theamount of filler was varied as shown in Table 1. The surface finish andtensile properties of these parts are shown in Table 2. ComparativeExample 1 displayed substantial slumping. Comparative Examples 2 and 3made good parts, but as is quite evident in Table 2, the surface finishis substantially rougher and on the order of 5 to 10 times rougher.

TABLE 1 Carbon Modulus Black Viscosity (G′) λ relaxation Width ofExample (wt %) (Pa · S) (Pa) time (s) Part (mm) 1 17.1 571 101500 1481.2 Comp. 1 16.5 531 60570 360 3.4 Comp. 2 20.0 656 328500 18.5 1.1Comp. 3 27.5 3982 1586000 17 0.9

TABLE 2 Tensile Tensile Carbon Surface Peak to Valley StrengthElongation at Strength Elongation at Example Black (%) roughness atHeight (μm) (XY) break (XY) (Z) Break (Z) 1 17.1 1.87 ± 0.15  6.2 ± 1.26.2 MPa 860% 7.9 MPa 840% Comp. 1 16.5 0.93 ± 0.46 N/A 7.7 MPa 730% 7.3MPa 770% Comp. 2 20.0 9.64 ± 0.47 32.9 ± 3.2 5.5 MPa 1930% 5.3 MPa 1640%Comp. 3 27.5 16.45 ± 5.75  50.42 ± 22.9 8.5 MPa 1050% 2.6 MPa 370%

1. A method of additive manufacturing comprising, (i) providing amaterial comprised of a prepolymer comprised of an isocyanate terminatedprepolymer and a filler in an amount such that the material has a shearstorage modulus C′ of 100,000 to 300,000 Pa measured at an oscillationrate of 1 Hz and a relaxation time of 20 seconds to 360 seconds, (ii)dispensing said material through a nozzle to form an extrudate depositedon a base, (iii) moving the base, nozzle or combination thereof whiledispensing the material so that there is horizontal displacement betweenthe base and nozzle in a predetermined pattern to form an initial layerof the material on the base, and (iv) repeating steps (ii) and (iii) toform a successive layer of the material adhered on the initial layer toform an additive manufactured part.
 2. The method of claim 1, whereinthe method further comprises repeating step (iv) such that a pluralityof successive layers are adhered and built up forming the additivemanufactured part.
 3. The method of claim 1, wherein the prepolymer hasa molecular weight average Mz of 10,000 to 500,000 g/mole,
 4. The methodof claim 1, wherein the material is shear thinning.
 5. The method ofclaim 4, wherein the material has a viscosity of at least 10,000centipoise at a shear rate of 5 s⁻¹ and a viscosity that is at least 10times less at a shear rate of 100 s⁻¹.
 6. The method of any one of thepreceding claim 1, wherein the prepolymer is comprised of a polyetherisocyanate terminated prepolymer.
 7. The method of claim 6, wherein theprepolymer is further comprised of an acrylate monomer, oligomer orprepolymer.
 8. The method of claim 1, wherein the filler is comprised ofa filler having an oil absorption number of at least 80 ccs/100 grams.9. The method of claim 8, wherein the filler has a product of the iodinenumber in mg/g multiplied by the oil absorption number in ccs/100 gramsof at least
 6000. 10. The method of claim 1, wherein the material is onecomponent and the prepolymer cures upon exposure to moisture.
 11. Themethod of claim 10, wherein the method is performed in gaseousatmosphere having a relative humidity of 10% to 90%.
 12. The method ofclaim 11, wherein the gaseous atmosphere is air.
 13. The method of claim1, wherein the prepolymer has a viscosity of 1000 centipoise to 30,000centipoise.
 14. The method of claim 1, wherein the filler is present inan amount of 15% to 18% by weight of the material.
 15. (canceled)